The present invention relates to an apparatus for controlling the fuel injection in engines that are able to perform stratified charge combustion. More particularly, the present invention pertains to an apparatus for controlling the fuel injection in stratified combustion engines that are provided with brake boosters, which use negative pressure to improve braking force.
In a typical engine, fuel is injected into an intake port from a fuel injection valve to charge a mixture of fuel and air to the associated combustion chamber in a uniform manner. An intake passage is opened and closed by a throttle valve, which is operated by manipulating an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of uniform mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
However, when performing uniform charge combustion, much negative pressure is produced by the throttling action of the throttle valve. This increases pumping loss, which is generated when the mixture is drawn into the combustion chamber from the intake port. To attempt to solve this problem, stratified charge combustion has been proposed. In stratified charge combustion, the throttle valve is opened wide, and fuel is supplied directly into each combustion chamber. This provides a mixture having a relatively low air-fuel ratio in the vicinity of the ignition plug. As a result, ignitability is enhanced.
Japanese Unexamined Patent Application No. 8-164840 describes an engine that performs stratified charge combustion. In this engine, each combustion chamber is provided with a uniform charge fuel injection valve and a stratified charge fuel injection valve. The uniform charge injection valve uniformly disperses fuel into the combustion chamber and the stratified charge injection valve injects fuel toward the vicinity of the ignition plug. When the engine load is small, fuel is injected from the stratified charge injection valve. Thus, the fuel is supplied in a concentrated manner about the ignition plug. The throttle valve is almost fully opened to perform stratified charge combustion. This improves fuel efficiency and decreases pumping loss.
This engine is also provided with a brake booster that increases the braking force thereby decreasing the required force for depressing of the brake pedal. The brake booster uses negative pressure, which is produced in the intake passage downstream of the throttle valve, as a drive source. In other words, negative pressure is communicated to the brake booster through a communicating pipe connected to the downstream side of the throttle valve. Negative pressure, which corresponds to the degree of depression of the brake pedal, acts on a diaphragm, which is incorporated in the brake booster, and increases the force actuating the brake.
However, in such an engine, pressure in the intake passage is increased during stratified charge combustion. That is, there is less vacuum pressure available. This may cause the negative pressure that actuates the brake booster to be insufficient. Accordingly, in the engine described in the above publication, the throttle valve is closed when the negative pressure that actuates the brake booster becomes insufficient. This guarantees the necessary negative pressure.
However, closing the throttle valve, for example, when the engine is idling, increases pumping loss and lowers the idling speed.
One of the ways to bring the lowered idle speed back to a target speed is to feedback control the amount of fuel injection, that is, to increase the fuel injection amount in accordance with the decrease of the idle speed. This feedback control, however, may have a response delay. For example, a delay in closing the throttle valve for producing necessary negative pressure for the brake booster quicken the pumping loss. In this case, the above described feedback control is too slow, and the engine speed is inevitably lowered and becomes unstable. If the engine speed is significantly lowered, the engine will stall.
The above described feedback control results in a lowered engine speed and reduced engine power not only when the engine is idling, but also when the vehicle is moving at a normal speed.